TRUE VOLCANOES. 315 



strata of the atmosphere. On descending from the central 

 plateau of Mexico (whose mean elevation is 7460 feet), 

 to the corn-fields of Yalladolid de Michuacan, to the charm- 

 ing lake of Patzcuaro, with the inhabited islet Janicho 

 and into the meadows around Santiago de Ario, which 

 Bonpland and I found adorned with the dahlias which have 

 since become so well known, we have not descended more 

 than nine hundred or a thousand feet. But in parsing from 

 Ario on the steep declivity over Aguasarco into the level of 

 the old plain of Jorullo, we diminish the absolute elevation in 

 this short distance by from a^O to 4250 feet 7 . The roundish, 

 convex part of the upheaved plain is about 12,790 feet in 

 diameter, so that its area is more than seven square miles. 

 The true volcano of" Jorullo and the five other mountains 

 which rose simultaneously with it upon the same fissure, are 

 so situated that only a small portion of the Malpais lies to 

 the east of them. Towards the west, therefore, the number 

 of hornitos is much larger, and when in early morning I 

 issued from the Indian huts of the Play as de Jorullo, or 

 ascended a portion of the Cerro del Mirador, I saw the black 

 volcano projecting very picturesquely above the innumerable 

 white columns of smoke of the "little ovens" (hornitos). 

 Both the houses of the Playas and the basaltic hill Mira- 

 dor are situated upon the level of the old non- volcanic, or, 

 to speak more cautiously, un-upheaved soil. Its beautiful 

 vegetation, in which a multitude of salvias bloom beneath 

 the shade of a new species of fan palm (CorypTia pumos), 

 and of a new alder (Alnus Jorullensis), contrasts with the 

 desert, naked aspect of the Malpais. The comparison of the 

 height of the barometer 8 at the point where the upheaval 



" My barometric measurements give for Mexico 1168 toises (7470 

 feet), Valladolid 1002 toises (6409 feet), Patzcuaro 1130 toises (7227 

 feet), Ario 994 toises (6358 feet), Aguasarco 780 toises (4^89 feet), for 

 the old plain of the Playas de Jorullo 404 toises (2584 fett) (Humboldt, 

 Observ. Astron, vol. i, p. 327, Nivellement Barometrique, No. 366 370). 



8 If the old plain of the Playas be 404 toises (2584 feet), I find for 

 the maximum of convexity of the Malpais above the sea-level 487 

 toises (3115 feet) ; for the ridge of the great lava-strearn 600 toises 

 (3838 feet); for the highest margin of the crater 667 toises (4266 feet): 

 for the lowest point of the crater at which we could establish the 

 barometer 644 toises (4119 feet). Consequently the elevation of the 

 summit of Jorullo above the old plain appeared to be 263 toises or 

 1682 feet. 



